Conventional high-pressure gases are typically delivered to process tools within semiconductor fabricators (e.g., chemical vapor deposition or CVD tools) by complex gas distribution systems. Flammable, pyrophoric, and toxic compressed gases such as silane, phosphine (PH.sub.3), and arsine (AsH.sub.3) (pure or mixed) typically require a multiplicity of equipment for safe storage and delivery. Such equipment typically includes a gas cabinet, one or more valve manifold boxes ("VMBs"), and gas isolation boxes ("GIBs"). The gas cabinet is typically positioned remotely from the process tool or the semiconductor fabricator itself. VMBs and GIBs are typically located locally within the fabricator. These systems are typically connected with gas piping or tubing and the functions of each sub-system, when coupled together, provide total gas management (e.g., purge, evacuation and process gas supply) for a particular flammable, pyrophoric, or toxic compressed gas.
At the tool level, because CVD processes run at medium vacuum levels (10.sup.-1 -10.sup.-4 torr), controlling the dopant partial pressure and overall pressure in the process chamber of the process tool becomes increasingly difficult when using dilute gas mixtures of such gases. The inclusion of balance gases such as helium or argon contributes additional gas molecules in the process chamber of the process tool, which results in higher operating pressures and reduced film deposition rates. Specifically, dilute fixed phosphine mixtures do not provide adequate dopant incorporation in a PSG film unless the flow of the PH.sub.3 mixture is increased significantly. However, increasing the flow of the dilute PH.sub.3 mixture has the effect of lowering the film deposition rate and negatively altering the process capability. In addition, there is the safety risk of a catastrophic release of gas with a high-pressure gas delivery system.
The use of sub-atmospheric gas (i.e., gas that is below 1 atmosphere) as a source gas is an alternative to high pressure gas delivery systems. Such a gas source virtually eliminates the risk of catastrophic gas releases, thus allowing the safe storage and use of such gases at 100% concentration.
Sub-atmospheric gas sources are typically stored locally in the fabricator close to the process tools for which they are intended. This minimizes pressure losses across the delivery path and, therefore, increases the deliverable quantity from the sub-atmospheric gas cylinder. Conventional gas cabinets configured for compressed gases typically do not provide adequate safety and are not capable of sub-atmospheric gas delivery or management. Gas cabinets configured for compressed gases provide neither the ability to pump purge the entire delivery path effectively, nor the ability to extract gases from the process tool to the cabinet itself. Additionally, conventional gas cabinets configured for compressed gases do not generally provide suitable capability to sense, prevent and mitigate the backflow of gases to the sub-atmospheric gas cylinder.
Lastly, conventional compressed gas cabinets do not provide sufficient detection of in-board leaks to the system and gas cylinder, mainly due to the fact the conventional systems for compressed gases are meant to operate at greater than 0 psig. Also, the limitations of conventional gas cabinets can not be overcome for sub-atmospheric gases by merely providing the additional functionality of equipment such as VMBs and GIBs. Providing the external functionality of VMBs and GIBs is not a practical option due to installation space restrictions, pressure drop restrictions, and, potentially, gas flow restrictions.